There is presently in the world a high production of thin cardboard matched on both faces with a plastic film such as polythene, sometimes also coupled to an aluminum film, so as to form a sandwich. Said sandwich sheet is widely used for producing many kinds of disposable containers among which the most important ones are those destined to contain new or long preservation milk.
The firms producing this type of thin sandwich cardboard usually have a high quantity of scraps due both to the big productions involved and to the strict quality standards to which said production is subjected. As a consequence of that, many paper mills have been arising in the world which became specialized in processing these amounts of scraps in order to obtain therefrom, after a suitable pulping process, a cellulose of normally high quality. Actually, paper mills that process the scrap-sandwich sheets mentioned above produce scraps too, that is the plastic film, combined or not with aluminum film, that remains after the cellulose fiber has been reclaimed.
The problem concerning the reuse of said scrap plastic film is very important as the amount of said scraps, which in a middle-sized paper mill is about 2000 tons per year, can only be destroyed in a very expensive manner while on the other hand, it could be a precious raw material especially to be used for injection molding.
First of all it is necessary to remember that the present technology for washing the different types of scrab plastic films available on the market is not suitable to be applied to the material in question; in fact the washing machines of the known art are based on the conviction that a very fast movement of the only washing blade (1400/1800 rotations per minute) is necessary to allow the water to be directed towards all the particles of the milled film. Actually, not only in the above mentioned washing machines a lot of energy is employed, which makes the economical character of the machine quite questionable, but also the cleaning result of each single plastic fragment cleared off from the paper and cellulose fibers adhering thereon before the process, is very disappointing. In fact, no more than 200 kg per hour of material are washed using at least 180 HP and the water discharged from the washing machine still appears cloudy and polluted by the cellulose, which presumably means that the cycle of mechanical separation of the fibers has not yet been completed.
After a washing process carried out using a washing machine of the known art and respecting an average production of about 200-250 kg per hour, it is possible to evaluate that the degree of residual pollution due to cellulose fibers in a plastic film containing about 20% of fiber when introduced in the machine, is not less than 3-5%.
What stated above is also valid when it is necessary to wash plastic material polluted for example by the presence of earth, as in the case of sheets used in agriculture, or of other pollutants.